This invention relates to the formation of water-soluble copolymers (the term "copolymer" is used herein to denote an interpolymer of two or more monomers) of alpha-beta olefinically unsaturated carboxyl monomers, and more particularly to a process, carried out in a single vessel (hence referred to as a "one pot" or "single pot" process), starting only with a lower carboxylic acid. As uncrosslinked or uncured water-based coatings, they dry to a water-resistant film but are easily removed with alkaline soap and water, or aqueous alkali. Water-based coatings are used as a primary film former for uses where alkali solubility is desired. In alkaline water and several solvents, resins formed by the one-pot process of this invention yield films which by choice of monomers and their proportions, may be clear, water-resistant, glossy, non-tacky, and adhere well. Moreover, these resins lend themselves to be tailored for applications in paper coatings, enteric coatings, pressure sensitive adhesives, cosmetic preparations, baking enamels, textile print pastes, non-woven fabrics, leather and allied fields. Some of the most promising uses appear to be those in which the resin functions as a binder.
Water-soluble thermosetting copolymers of a lower carboxylic acid, a N-methylol acrylamide (or methacrylamide), and certain lower alkyl acrylates (or methacrylates), and a method for making them are disclosed in U.S. Pat. No. 3,007,887 to Henry J. Essig. Each of the monomers and a lower aliphatic alcohol solvent is charged, along with a peroxide initiator, into a glass lined reaction vessel and the polymerization is carried out at reflux temperature.
From a production point of view, it is undesirable to prepare each of the monomers separately. It is more economical to carry out the reaction starting only with the most easily available raw materials, concurrently use solvent, catalyst and initiator, along with inhibitors and accelerators if they are deemed necessary, and to run the polymerization in a one pot process. Except for the competing reactions which might interfere with formation of the desired polymer, this would appear to be quite feasible. It is not. To begin with, the presence of an N-methylol acrylic amide monomer in a one pot process, mixed with acrylic or methacrylic acid, an acrylate and a methacrylate, does not result in the polymer disclosed in the patent. Interfering side reactions dictate that the amide monomer be excluded from the reactants for a one pot process.
As is well known, acrylic acid and methacrylic acid for commercial purposes typically contain a significant amount of inhibitor. It is necessary to use sufficient initiator to overcome the effect of the inhibitor for polymerization to occur, and in the presence of the acid esterification catalyst, side reactions such as transesterification or, ionic decomposition of the peroxide initiator without formation of free radicals, may occur, the probability of occurrence of which militate against the use of a one pot concurrent esterification and polymerization reaction.
It is also known that the heterolytic decomposition of peroxides would be expected to be promoted by electronically dissimilar substituents on the peroxide group, and by a polar medium, and should be susceptible to acid catalysis (see "O-O Heterolysis: Intramolecular Nucleophilic Rearrangement" in ORGANIC PEROXIDES, by Alwyn G. Davies, Butterworths, London 1961).
Still further, it is particularly significant that, in view of the many ways available to esterify an alpha-beta unsaturated monocarboxylic acid, I known of only one method which economically lends itself to concurrent esterification and polymerization.
A further complication is that esterification of the carboxylic acid monomer results in the formation of water which, though eventually removed by refluxing the reaction mass in my novel process, nevertheless may adversely affect both the progress of the desired polymerization reaction, and the resulting products of polymerization.
Still another complication is that it is known that the presence of water in a polymerization in alcohol, may accelerate the reaction and raise the molecular weight much higher than in an anhydrous system. Particularly since the starting material is acrylic acid or methacrylic acid, it is quite unexpected that the presence of water and the presence of the acid esterification catalyst at reflux temperatures still produces a highly desirable copolymer.